We have learned a great deal recently about the mechanism and biology of a class of small non-coding RNAs called microRNAs. This remarkable class of RNAs constitute 1 % of the genes in the human genome, and they repress the expression of protein-coding genes. This is achieved in animal specieis primarily by attenation of protein synthesis from messages which contain complementary sequence in their 3' untranslated regions. Although it is difficult to estimate the extent of microRNA regulation, from 4 - 20% of protein-coding genes might be directly controlled by microRNAs. Genetic studies in model organisms support the notion that microRNAs play select roles in cell and organismal biology. We are interested in understanding how microRNAs specifically inhibit their target genes and the biological consequences of this regulation. To this end, we developed a method to detect individual microRNAs in whole Drosophila tissues and embryos by in situ hybridization. We will evaluate the expression patterns of the 78 known Drosophila microRNAs during eye development. The upstream regulatory pathways that regulate microRNA expression in the eye will be evaluated, and correlated to known biological functions of these pathways. We will use microRNA gene mutagenesis to determine if microRNAs are involved in these biological processes. A combination of in silico and molecular genetic approaches will be used to identify and characterize direct targets of microRNA repression, and to investigate the mechanism(s) governing the specificity of microRNA- mediated translational repression. Our goal is to decipher the rules of microRNA regulation regarding target and biological specificity in diverse tissues of the body. The combined effects of microRNAs may affect the expression of many human genes, and misregulation of microRNAs may well underlie complex disease phenomena such as cancer susceptibility and progression.